A power regenerative converter is disposed between an inverter device for variable speed controlling a three-phase induction motor and a three-phase AC power supply and a reactor is provided between the three-phase AC power supply and the power regenerative converter. The power regenerative converter regenerates, in a three-phase AC power supply, an induced electromotive force generated in a speed reduction of a three-phase induction motor (hereinafter referred to as a motor). When a speed of the motor is reduced, a current obtained by the induced electromotive force thus generated flows into both terminals of a smoothing capacitor in the power regenerative converter. When a regenerative transistor of the power regenerative converter is turned ON, a regenerative current flows from the smoothing capacitor into a power supply.
Thus, a difference between a voltage of the smoothing capacitor of the power regenerative converter and a source voltage is utilized to cause a current to flow with a current limitation by the reactor. If a phase of the regenerative transistor to be turned ON is taken erroneously, therefore, the difference in a voltage is increased and a large current suddenly flows so that an apparatus might be stopped or broken down. For this reason, a DC bus voltage value is compared with a regeneration starting voltage value, and a regenerating operation is started when the DC bus voltage value is higher than the regeneration starting voltage. For a command to be given to each regenerative transistor during the regenerating operation, moreover, an ON/OFF signal of the regenerative transistor which is generated based on a phase of a detected three-phase line voltage is used to carry out the power regenerating operation (for example, see Patent Document 1).
Patent Document 1: JP-A-2000-253686 Publication (Paragraphs 0018 to 0021, FIG. 1)